This invention relates to optical transmission networks, to nodes for use in such networks, to link selectors for use in such nodes, to controllers for link selectors, to protection path sharing arrangements for optical transmission links, to methods of transmitting data over such networks, to software for managing such networks, and to software for determining configuration of secondary paths in such networks.
Optical transmission systems are often constructed with a fault recovery mechanism so that if there is a complete loss of transmission capability, e.g. from a cut in the fibre, or a failure in the transmission path for any other reason, the traffic can be reallocated to other physically diverse routes. Known fault recovery mechanisms can be classed in two categories, firstly mesh restoration schemes, and secondly linear or ring protection methods. The term xe2x80x9cprotectionxe2x80x9d implies a very fast recovery. The term xe2x80x9crestorationxe2x80x9d implies a slower recovery with correspondingly more disruption to traffic.
Mesh restoration method speeds may be in the order of one to fifteen minutes, but are relatively efficient in terms of the amount of bandwidth set aside for recovery. Linear or ring protection methods may complete their operation in the order of ten to sixty milliseconds, but are less bandwidth efficient, typically requiring fifty to seventy percent of network capacity to be set aside for protection.
Restoration may take place at various layers of the well-known OSI model. There are advantages to conducting the restoration at the lowest feasible level, to minimise congestion that can be caused by delays and retransmissions of traffic. Restoration at the optical level, by switching a signal to another optical path, can be done either by optical switching, or by converting into the electrical domain, and switching in the electrical domain, if necessary, using demultiplexing if the bitrate of the optical signal is too high for electrical switching.
Mesh restoration operates by identifying several different alternative paths through the mesh to the destination node according to availability of bandwidth on these paths. The traffic is divided between these alternative paths, and recombined at the destination node, at the far side of the fault. It is bandwidth efficient because the amount of bandwidth on each link set aside for protection purposes need only be a fraction of the bandwidth of each working transmission path. However, it is usually slow to operate because for a given fault in the mesh, the alternative routes need to be determined, often by a central controller, and it may take time for the numerous nodes in the different paths to be configured. Then the traffic can be divided appropriately between the various protection paths to the destination node.
The software for controlling such restoration may be complex, and may require manual intervention, for a large mesh network.
Linear or ring protection methods involve providing a preconfigured (and often dedicated, though it can be shared) protection path for each link. Since the working path will normally take the shortest route between nodes, the protection path will normally involve more links than the working path. Accordingly, over the entire network, more protection path bandwidth needs to be provided than working bandwidth, and so working bandwidth may be as low as twenty to fifty percent of total bandwidth. This is expensive but it enables the protection path to be switched with a minimum of processing. Ideally, as soon as the fault is detected, a signal is sent to a switch at each end of the protection path to bring the protection path into operation. In this case there are only two switches to operate, there is no path determination, or splitting up and recombination of the signal. The process needs no central control, and can therefore happen very quickly, in the order of ten to sixty milliseconds.
Many variations have been proposed, within each of these two categories, attempting to achieve a physical layer fault recovery scheme that is both fast, and offers efficient use of bandwidth. In the category of protection schemes, one known option is to provide a pre-configured, dedicated protection path for each working path. A simple example is shown in FIG. 1.
Another example is known from U.S. Pat. No. 5,159,595, (Nortel Networks reference RR1110) showing a bidirectional ring, and a capability at nodes next to a fault, to couple a working path onto the path going in the other direction around the ring, to create a folded loop. Another example making use of an optical switch to enable a protection ring to bypass a node is shown in WO97/09803 (Nortel Networks reference RR2473).
Another proposal for a protection scheme is shown in patent publication WO9923773 (Nortel Networks reference RR2258) involving an optical network made up of working paths in the form of SONET rings. On a link where protection paths for these SONET rings overlap, instead of providing two fibers for the two protection paths, a single fiber is shared by the two protection paths. An optical selector needs to be provided at the nodes at each end of the link. At these nodes, the protection paths pass through ADMs (Add Drop Multiplexers), where the protection path would be converted to the electrical domain to enable the working path to be switched onto the protection path. Although offering a bandwidth saving, the disadvantage of the optical degradation introduced by the optical selector, or the cost of additional equipment to overcome this, have tended to outweigh the benefit. Accordingly, this proposal has not been adopted on a commercial scale. More recently, sharing of protection bandwidth has been achieved in a different way, by optical shared protection rings (OSPR). This technique has been adopted for commercial use.
FIG. 2 shows an OSPR. The protection ring extends over several links in the network. On each link, the single ring can effectively replace several dedicated protection paths. Each of the protection paths still follows a preconfigured route around part of the ring. Accordingly, there is no need to determine a route and divide up the traffic, and so the speed of recovery is relatively good. As this protection ring can be shared between multiple protection paths, there is an improvement in bandwidth efficiency over using dedicated paths, to a ratio in the illustrated example of 1 protection path to 1 working path, i.e. 50% bandwidth efficient.
An OSPR scheme has been published in xe2x80x9cTechnical Digest of the Optical Fiber Communication Conference and the International Conference on Integrated Optics and Optical Fiber Communication, San Diego, Calif., Feb. 21-26, 1999, paper entitled Availability analysis of optical shared protection rings for long haul networks, by Philippe Neusy and Richard Habel, pages 176/TuL5-1 to 179/Tul5-4xe2x80x9d.
OSPR has been regarded as an optimum solution that can be tailored to suit the networks of working paths arranged in either rings or meshes. Efforts to achieve further bandwidth efficiency improvements have therefore focussed on how to arrange the protection rings to be shared between more protection paths on the rings.
References to optical networks in this document are not intended to be limited to all-optical, but are intended to include for example networks making use of optical transmission and electrical domain switching.
References to nodes in the network can encompass add/drop points where traffic is added or dropped from the network. They can also encompass re-routing junctions where no traffic is added or dropped from the network. References to links between nodes are intended to encompass lines between add/drop points, or re-routing junctions, which may include a proportion of a span, or one or more spans. Spans are optical paths between electrical regeneration or optical amplification points.
References to working paths are intended to encompass end to end paths set up over multiple networks, or over multiple links in a network, or over parts of rings in a network. References to a mesh of working paths are intended to encompass any arrangement of paths beyond a single path, including combinations of rings.
References to secondary paths are intended to encompass protection paths and any path provided as an alternative or back up, in case of failure of a working path. The term xe2x80x9ctranspondersxe2x80x9d is used throughout and is intended to encompass transceivers, remodulators, regenerators and any device for receiving an optical signal and transmitting the signal onwards at the same or a different wavelength. They may be used for signal regeneration, for enhancing reach, or for wavelength conversion. They may also be used to carry out performance monitoring, and couple network signalling to the optical signal.
According to a first aspect of the invention, there is provided an optical network comprising a number of nodes and links between the nodes, a mesh of working paths provided over the links and nodes, a number of secondary paths provided over the links and nodes, each for use in case of a failure in a different part of one of the working paths, a first shared protection ring being provided for shared use by the secondary paths, and being arranged to overlap on a given one of the links with one or more of the secondary paths that do not share that first shared protection ring; and a link selector to select which of the first shared protection ring or the one or more secondary paths routed to overlap on the given link, is allocated to the given link at any time.
An advantage of further bandwidth efficiencies can arise from the recognition that two forms of overlap of protection paths, by link selector and by shared ring, previously seen as alternatives, can in fact be combined.
Since the link selectors can be controlled locally, and essentially independently of the shared ring, the greater bandwidth efficiency can be achieved in a way that is more scalable to more complex networks, than is the known optical shared ring alone. Also, some of the apparent disadvantages of the link selector, such as the above mentioned degradation of the optical path of the shared link, can be reduced when the link selector is used in combination with a shared ring.
Furthermore, link selectors can be added to existing optical shared rings without needing major reconfiguration of the rings, since the link selectors can be controlled locally, and essentially independently of the shared rings.
By using a link selector for a pre-determined route, rather than dynamic routing of secondary paths, a number of advantages are obtained. The speed penalty of dynamic routing and the complex centralised control of bandwidth allocation can be avoided. Instead, the selector can be controlled locally and so can operate rapidly. The local control, which can be essentially independent of the switches used to set up the secondary path through the network, also makes the technique scalable to larger, more complex networks without an increase in complexity of control.
Preferably, any overlapping secondary paths and any overlapping shared rings being arranged such that any two of them do not overlap more than once with each other.
This condition ensures that a secondary path still exists for every working path in the event of any single fault. This condition can be maintained either for all secondary paths, or just a subset, e.g. those related to higher priority working paths.
Preferably the first shared protection ring comprises a first optical switch for coupling one or more of the secondary paths to the first shared protection ring, the optical switch being located adjacent to the link selector, on the first shared protection ring.
An advantage arises from having an optical switch on the ring next to the link selector rather than an electrical switch. The electrical switch needs an integral optical transmitter, (integral because separation of the optical transmitter would involve taking high speed electrical signals out of an integrated package, which is costly or technically impractical) and the link selector cannot easily be integrated before the transmitter. Accordingly, the link selector will be in the optical path and will degrade the optical path or will need a separate amplifier, transponder or transmitter, which can be so expensive as to outweigh other advantages. These difficulties can be reduced by providing an optical switch, since such a switch needs no integrated transmitter. Instead, having the optical switch and the link selector adjacent offers opportunities for sharing components such as transponders, to improve the trade off between cost, and optical performance. This advantage can apply whether the link selector is formed by an electrical or an optical switch.
Preferably one of the nodes of the network comprises a transponder, at least part of the transponder being located on a link side of the link selector. This enables fewer transponders to be used, and means that optical degradations introduced by the link selector may not reach the link. Thus cheaper components can be used in the link selector. This is particularly significant at higher speeds or for longer reach systems, where the cost of a transponder can be the dominant cost of a node or even of the whole network.
Preferably the network comprises a first optical switch for coupling one or more of the secondary paths to the first shared protection ring, the optical switch being located adjacent to the link selector, on the first shared protection ring, and a transponder for converting optical wavelength, to launch an optical signal onto the first shared protection ring, the transponder being located on the coupled secondary path before this path enters the first shared protection ring.
If the transponder is instead located off the shared protection ring, optical degradation around the ring may be worse, but it is possible to arrange for through traffic on the shared protection ring to bypass the transponders completely. This can make the shared ring cheaper, and more easily adaptable to different protocols, and thus easier to upgrade in the future.
Preferably the one or more secondary paths routed to overlap on the given link use a second shared protection ring which overlaps on the given link with the first shared protection ring. This more specific case of selecting between overlapping shared protection rings may yield most bandwidth efficiency improvements. It involves an overlap of two or more shared resources, and so can maximise the overall amount of sharing of the given link. Shared rings are expected to become more widely used in optical networks than other mechanisms.
Preferably the link selector comprises a second optical switch. This may be simpler, cheaper or faster than an electrical switch.
Preferably the link selector is-controllable on the basis of levels of priority of different secondary paths. This may enable different levels of protection or survivability for different working paths to be offered.
Preferably a signalling arrangement is provided between the link selector and a node at the far end of the given link so that the same one of the overlapping secondary paths is selected at both ends of the given link.
This is better than the alternative of having the secondary path inform the link selector at each end of the given link, without any signalling between the respective link selectors. In such a case it would be impossible to resolve conflicting secondary path requests at opposite ends of the given link, and thus misconnections could result. A signalling arrangement can resolve or avoid such conflicts.
Another aspect of the invention provides a node for use in the above network, at a near end of the given link, as claimed.
Another aspect of the invention provides a node for use in the above network, at a far end of the given link, as claimed.
Another aspect provides a link selector as claimed.
Another aspect provides a controller for controlling the link selector, as claimed.
Another aspect provides a method of transmitting traffic using the above network.
Another aspect provides a node for use in an optical network, the network comprising a number of other nodes and links between the nodes, a mesh of working paths provided over the links and nodes, a number of secondary paths provided over the links and nodes, each for use in case of a failure in a different part of one of the working paths, some of the secondary paths having pre-determined routing so as to overlap on a given one of the links, the node being for use at one end of the given link, and comprising: a selector for the given link to select which of the secondary paths routed to overlap on the given link, is allocated to the given link at any time, and, a transponder, the transponder being located on the link side of the link selector.
Another aspect provides a protection path sharing arrangement as claimed.
Another aspect provides software for determining a configuration of secondary paths in the above network, as claimed.
Any of the preferred features may be combined with any of the aspects set out above as would be apparent to a skilled person.
Other advantages will be apparent to a skilled person, particularly in relation to any further prior art other than that discussed above.